The present invention relates to a semiconductor device and a method for fabricating the same and, more particularly, to a semiconductor device having a capacitance insulating film composed of a high dielectric material or a ferroelectric material and a method for fabricating the same.
In recent years, a memory cell portion in a semiconductor memory device has been scaled down increasingly with design miniaturization.
A memory cell portion in a dynamic random access memory (DRAM) device which is among semiconductor memory devices is composed of a capacitor for storing charges and a pass gate transistor for selectively making the capacitor accessible. In a typical DRAM device, even if the plan size of a memory cell portion is reduced and the projected area of a capacitor is thereby reduced, the storage capacitance of the capacitor cannot be reduced for lower power consumption and for the prevention of a soft error.
The storage capacitance of a capacitor is directly proportional to the relative dielectric constant of a dielectric film composing a capacitance insulating film and to the area of the dielectric film interposed between opposing electrodes, while it is inversely proportional to the thickness of the dielectric film. If the thickness of the dielectric film is reduced for an increase in the storage capacitance of the capacitor, a leakage current in the capacitor increases and therefore a refresh cycle time in the memory cell portion should be increased, which leads to increased power consumption. This places a limit on a reduction in the thickness of the dielectric film used as the capacitance insulating film.
In recent years, the use of a dielectric material having a high relative dielectric constant for a capacitance insulating film has been studied vigorously in an attempt to increase the storage capacitance of a capacitor. As exemplary materials having high relative dielectric constants, metal oxides such as an aluminum oxide (Al2O3) and a tantalum pentoxide (Ta2O5) and composite metal oxides each having a perovskite crystal structure represented by the general formula ABO3 (where each of A and B is a metal element), such as barium strontium titanium oxide ((Ba,Sr)TiO3, hereinafter referred to as BST), lead zirconium titanium oxide (Pb(Zr,Ti)O3, hereinafter referred to as PZT), and strontium bismuth tantalum oxide (SrBi2Ta2O9, hereinafter referred to as SBT) have been studied in detail.
In forming a thin film from such a high dielectric material, a chemical reaction is used normally in most cases. Since the thin film is formed in an oxidizing atmosphere, if silicon (Si) used commonly is used for an electrode, the silicon is easily oxidized to form a silicon dioxide film having a low relative dielectric constant so that it is difficult to increase the capacitance value of the capacitor.
For the electrode of a capacitor using a high dielectric film as a capacitance insulating film, therefore, a precious metal, a refractory metal, or the like is used. If BST is used for the capacitance insulating film, e.g., ruthenium (Ru), ruthenium dioxide (RuO2), platinum (Pt), iridium (Ir), or the like is used for the electrode. If a ferroelectric film composed of SBT or PZT is used as the capacitance insulating film, platinum (Pt), iridium (Ir), iridium dioxide (IrO2), or the like is used for the electrode.
As an electrode for a capacitor using a high dielectric material or a ferroelectric material has been miniaturized increasingly in recent years, the electric characteristic or reliability of the capacitor is more greatly influenced by step coverage in the formation of a capacitance insulating film. Under such circumstances, research on the formation of a capacitance insulating film by chemical vapor deposition, which is excellent in step coverage, has been pursued. In particular, step coverage is further improved if a temperature range in a region where the rate of a surface reaction is determined is used in chemical vapor deposition.
However, the foregoing conventional method for fabricating a semiconductor device by chemical vapor deposition has the problem that the density or composition ratio of a film to be formed varies depending on the surface state of a film underlying the film to be formed. If the density of the film varies, e.g., a desired thickness cannot be achieved or the thickness of the film varies depending on the surface state of the underlying film. If the composition ratio of the film varies, on the other hand, desired properties including a relative dielectric constant cannot be obtained.